Head mountable device

1. A mobile device for being held by a mobile device holder of a head-mountable display device, the head-mountable display device comprising: a frame to be mounted onto a user's head, the frame defining an eye display position which, in use, is positioned in front of an eye of the user; a reflective element mounted with respect to the eye display position, the reflective element comprising a reflective surface which, in use, is visible to the eye of the user; and a mobile device holder configured to hold a mobile device at a location and orientation on the frame; wherein: the reflective surface is configured to reflect light incident upon the reflective surface so as to form a reflected image; and the location and orientation in which the mobile device is held relative to a location and orientation of the reflective surface are such that, in use, light arriving from a screen of the mobile device held in the mobile device holder is reflected by the reflective surface so that a reflected image of at least a portion of the screen is visible to the user, the mobile device comprising: a screen; an image generation unit operable to generate an image for being displayed on the screen and reflected in the reflective surface of the reflective element of the head-mountable display device; an image warp compensation unit operable to warp the generated image so as to compensate for a warped appearance of the image when it is reflected, the image warp compensation unit warping the generated image on the basis of one or more parameters related to curvature of the reflective surface of the reflective element of the head-mountable display device, wherein the screen is operable to display the warp-compensated image; a camera operable to capture an image comprising a reflected image of the eye of the user reflected by the reflective surface of the reflective element and comprising an image of a portion of the user's visual environment; an eye-tracking unit operable to determine a first portion of the captured image corresponding to the reflected image of the eye of the user and to perform eye-tracking processing on the first portion; and a location and mapping unit operable to determine a second portion of the captured image corresponding to the image of the portion of the user's visual environment and to perform location and mapping processing on the second portion.

2. A head-mountable device, comprising: a frame to be mounted onto a user's head, the frame defining an eye-tracking reflection position which, in use, is positioned in front of an eye of the user; a reflective element mounted with respect to the eye-tracking reflection position, the reflective element comprising a reflective surface which is located and orientated so as to, in use, reflect light arriving from the eye of the user; and a mobile device holder configured to hold a mobile device at a location and orientation on the frame; wherein: the reflective surface is configured to reflect light incident upon the reflective surface so as to form a reflected image; the location and orientation in which the mobile device is held relative to the location and orientation of the reflective surface are such that, in use, the light arriving from the eye of the user is reflected by the reflective surface so that a reflected image of the eye of the user is within a field of view of a camera of the mobile device; and the location and orientation in which the mobile device is held are such that, in use, a portion of the user's visual environment is within the field of view of the camera of the mobile device.

3. A device according to claim 2 , wherein the reflective element is semi-transparent.

4. A device according to claim 3 , comprising: a non-transparent shutter element movably mounted on the frame and operable to be moved into a position in which at least a portion of a surface of the reflective element other than the reflective surface is overlapped by the non-transparent shutter element, so that an amount of light incident upon the at least a portion of the surface of the reflective element is reduced.

5. A device according to claim 2 , wherein: the head-mountable device comprises a reflective element transparency control unit; and the reflective element comprises a liquid crystal shutter element operable to alter transparency of the reflective element in response to an instruction issued by the reflective element transparency control unit.